Several ribonucleases of the RNase A family function as antibacterial, anti-parasitic and anti-viral agents. In this work, we have shown that mRNAs encoding five of the six known human ribonucleases of the RNase A family are expressed in cultured human monocytes, and that ribonucleases are released by adherent monocytes in culture. Using a polyclonal antiserum prepared against recombinant protein, we have detected one of these ribonucleases, RNase 4, in lysates of normal human peripheral blood monocytes, but not granulocytes or lymphocytes, by Western blotting. Subcellular localization by immunoelectron microscopy demonstrated the presence of RNase 4 in the cytoplasmic granules of isolated monocytes. Interestingly, mRNA encoding RNase 4 could not be detected in freshly isolated monocytes, emerging only after 16 h in culture, suggesting the possibility of de novo protein synthesis in association with monocyte differentiation.